In general, an electronographic image forming apparatus such as a copying machine, a printer, and a facsimile machine may include an image forming mechanism for forming a toner image on a sheet of recording medium, and a fixer to fix the toner image on the recording medium.
A fixer generally includes a fixing sleeve having a heating layer, a heating source to heat the fixing sleeve, a pressurizer pressing the fixing sleeve and forming a fixing nip with the fixing sleeve. When the sheet passes through the fixing nip, the image is fixed with heat from the fixing sleeve and pressure from the pressurizer.
Electromagnetic induction heating fixers are widely used in image forming apparatuses to shorten the warm-up time of the fixer and to save energy. An example electromagnetic induction heating fixer include a fixing sleeve having a hollow center and a heating layer; a fixing assist roller provided inside the fixing sleeve; a coil unit facing the fixing sleeve (induction heating source); and a pressurizer pressing the fixing assist roller via the fixing sleeve (outer pressurizer). At a contact position of the fixing sleeve and the pressurizer, a fixing nip is formed. The fixing assist roller presses the fixing sleeve at the fixing nip from the inside. An elastic insulating layer is formed over a core metal in the fixing assist roller.
When a high-frequency alternating current is applied to the coil unit, an alternating magnetic field is formed around the coil unit. The magnetic field causes an eddy-current in the heating layer of the fixing sleeve. Where the eddy-current is generated, Joule heat is generated due to electric resistance of the heating layer. The Joule heat heats the whole fixing sleeve.
Another electromagnetic induction heating fixer includes a fixing roller having a heating layer, an elastic layer, and an insulating layer that are bonded together through a plasma process, etc. Because of the elastic layer or the elastic insulating layer, the fixing nip may be sufficient. Further, heat loss, which is the heat transferred from the fixing sleeve to the fixing assist roller, is reduced because of the insulating layer or the elastic insulating layer.
Another electromagnetic induction heating fixer includes a fixing roller, a pressing roller, and a flux generator located inside the fixing roller. The flux generator is located across the width of the fixing roller. The Curie point of the flux generator is lower at the region near both ends thereof than at a center region thereof in the width direction of the fixing roller. Heating efficiency is decreased when a temperature of the flux generator exceeds the Curie point. However, the fixing roller may be overheated before the heating efficiency of the flux generator is decreased.
One example pressing roller or insulating roller, which may be included in a fixer, includes an insulating layer having a concave-convex circumference surface to enhance insulating properties.
In general, an image forming apparatus is configured to handle various width size of recording media (e.g., sheets). Further, even for a same sized sheet, the image forming apparatus handles sheets having different widths when a longer side of the sheet is parallel to a sheet transfer direction and a shorter side of the sheet is parallel to the sheet transfer direction.
When a sheet having a smaller width passes a fixing nip formed between a fixing roller and a pressing roller, more heat is lost from a region of the fixing roller where the sheets pass (e.g., center region) than from other regions where sheets do not pass (e.g., regions near both ends). Therefore, a fixing temperature at the center region thereof becomes lower than the temperature at regions near both ends thereof. This phenomenon is clear when images on the sheets having smaller widths are continuously fixed.
Therefore, if the fixing temperature across the fixing roller in width direction thereof is controlled based on the temperature at the center region thereof, the temperature at regions near both ends thereof tends to excessively rise (overheated region). When a sheet having a larger width in a width direction thereof is fixed by the fixer in the above condition, a fixing failure (e.g., hot offset) occurs on a region of the sheet corresponding to the overheated region of the fixing roller. Further, the fixing roller may be thermally damaged if the temperature at the overheated region exceeds an upper temperature limit of the fixing roller.
However, if the fixing temperature across the fixing roller in width direction thereof is controlled based on the temperature at the overheated region (regions near both ends), the fixing temperature at the center region decreases. When a sheet is fixed by the fixer in the above condition, a fixing failure (e.g., cold offset) occurs on a region of the sheet corresponding to the region of the fixing roller having a lower temperature (center region).
Further, when a paper jam occurs in a sheet transport path in the image forming apparatus, the fixer may be accidentally stopped. In such a case, a region of the fixing roller facing a flux generator (induction heater) may be overheated in a short period before the flux generator is turned off. As a result, the fixing roller and a component of the flux generator may be thermally damaged.